Add PostgreSQL notifier (#2739) (#2824)

* The user is required to specify a table name and database connection
  information in the configuration file.

* INSERTs and DELETEs are done via prepared statements for speed.

* Assumes a table structure, and requires PostgreSQL 9.5 or above due to
  the use of UPSERT.

* Creates the table if it does not exist with the given table name using
  a query like:

    CREATE TABLE myminio (
        key varchar PRIMARY KEY,
        value JSONB
    );

* Vendors some required libraries.

cmd/notify-redis.go

@@ -37,7 +37,8 @@ type redisConn struct {
 	params redisNotify
 }
 
-// Dial a new connection to redis instance at addr, optionally with a password if any.
+// Dial a new connection to redis instance at addr, optionally with a
+// password if any.
 func dialRedis(rNotify redisNotify) (*redis.Pool, error) {
 	// Return error if redis not enabled.
 	if !rNotify.Enable {